So I'm playing a sorcerer and I realized I might have the spell slots understood wrong.

For simplicity's sake, say I only have one level 1 spell slot.
After a long rest, do I have to prepare one spell (say it's magic missile) that I can use during the day (meaning throughout the day, I can use only magic missile), or can I just use absolutely any spell as long as it fits the criteria (i.e. it's only one level 1 spell)?

2 Answers
2

Sorcerers don't prepare spells

The premise of the question isn't correct. A Sorcerer does not prepare spells after a long rest. Starting from the 1st level, a Sorcerer knows two spells. You choose them in the process of the character creation. See the Spellcasting paragraph of the Sorcerer class, PH page 101:

You know two 1st-level spells of your choice from the Sorcerer Spells list.

You can't change these spells after a long rest. A long rest resets your spell slots, not the list of known spells:

You regain all expended spell slots when you finish a long rest.

You spend a spell slot in the very moment when you cast something, not when you prepare or learn spells (this is true for all spellcaster classes). You can cast the same spell more than once, providing you have spell slots available. See Can a spell be prepared once and cast multiple times?

And vice versa - if you know several spells having only one suitable spell slot, you can cast any known spell but only once.

Knowing, Preparing, and Casting are not the same thing...

While the spell slots progression is same1 for each type of Spellcaster2 (full, half, third), what spells they know and prepare vary by class.

Druids, Clerics, and Paladins know every spell on their class list, and prepare a subset of them each day. Some paths have a number of spells (potentially off-class) that are always prepared.

Wizards know a subset of their class list as contained in their spellbook; given time, the spellbook can include the entire set of wizard spells. They prepare a limited number of those spells each day.

Sorcerers, Bards, Arcane Tricksters, and Eldritch Knights know a subset of their class list and have all of them prepared at all times.3

All spellcasters cast spells in the same way: select a spell from their list of prepared spells, select an available spell slot of the appropriate level, and then cast the spell.

Spells remain prepared until the next long rest, when the character can shuffle spells around - casting doesn't affect the list of prepared spells. A character with four spells prepared and two slots could cast the same spell twice, or two different spells (once each).

1Disregarding multiclassing, for simplicity's sake.

2Despite similarities, a Warlock's Pact Magic is not Spellcasting. Again, I'm leaving it out for simplicity's sake.

3Technically, they don't prepare at all, but it's easier to explain if you assume they're all always "prepared".